Project Abstract Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide with over 600,000 cases diagnosed annually. Because currently approved treatment options for HNSCC are toxic, nonspecific and largely ineffective, 5-year survival rates have remained around 50% for the past several decades. The genetic landscape of HNSCC was recently published by The Cancer Genome Atlas which found that nearly 10% of tumors harbor mutations in the CASP8 gene. CASP8 encodes the procaspase-8 zymogen, which is a known regulator of cell death pathways in response to external cell death inducing stimuli. Recently published work from the Johnson lab indicates that procaspase-8 mutant proteins inhibit the induction of apoptosis in response to the death ligand, TRAIL, and our preliminary data suggests that procaspase-8 mutations confer increased resistance to the two most common therapies used to treat HNSCC, cisplatin and radiation. Additional preliminary findings suggest that procaspase-8 mutations render cells exquisitely sensitive to combined DNA damage inducing therapy and DNA damage response inhibition. Here, I hypothesize that procaspase-8 mutations alter the mechanism of cell death induced in response to DNA damage and enhance responsiveness to inhibitors of the DNA damage response pathway, creating a new therapeutic opportunity in HNSCC. First, I will determine whether procaspase-8 mutations alter the mechanism of cell death induced by DNA damage by examining activation of different cell death pathways and performing mass spectrometry to identify differential protein-protein interactions following treatment with cisplatin or radiation. Second, I will interrogate the efficacy of combined treatment of procaspase-8 mutant cells with cisplatin or radiation, and inhibitors of the DNA damage response pathway. Third, I will perform an unbiased genetic screen to elucidate novel synthetic lethal interactions with procaspase-8 mutations. Finally, I will test the preclinical relevance of these therapeutic strategies in well-characterized patient-derived xenograft (PDX) models of HNSCC. Together, these studies will further our understanding of the functional impact of procaspase-8 mutations in HNSCC, while providing a foundation for improved therapeutic strategies to treat tumors harboring these mutations.